One of the strengths of oxygen sensitive EPR materials, is that they can be implanted into tissues for use in monitoring oxygenation over days to weeks. One difficulty with such long term experiments (or with implanting in patients), is that the precise local of the material is not immediately evident. In order to overcome this problem, we are assessing the use of non-invasive MR imaging as a method of localizing the EPR implants. All of the EPR materials studied to date have a magnetic susceptability which makes them appear dark on gradient echo MR images. Conversely, with imaging protocols which minimize susceptability effects (short TE spin echo sequences), the material is minimally visible. In this project, we are optimizing imaging paramaters for either increasing or reducing the visibility of EPR materials in non-invasive MR exams.